Non-volatile memories are useful storage devices due to their ability to maintain data absent a power supply. One class of variable resistance materials for use in non-volatile memory cells are phase change materials, such as chalcogenide alloys, which are capable of stably transitioning between amorphous and crystalline phases. Each phase exhibits a particular resistance state and the resistance states distinguish the logic values of a memory element formed with such materials Specifically, an amorphous state exhibits a relatively high resistance, and a crystalline state exhibits a relatively low resistance.
A conventional variable resistance memory implemented as phase change memory element 1, is illustrated in FIGS. 1A and 1B, and often has a layer of phase change material 8 between first and second electrodes 2, 4. The first electrode 2 is arranged within a dielectric material 6. The phase change material 8 is set to a particular resistance state according to the amount of current applied between the first and second electrodes 2, 4. To obtain an amorphous state (FIG. 1B), a relatively high write current pulse (a RESET pulse) is applied for a first period of time through the phase change memory element 1 to melt at least a portion 9 of the phase change material 8 covering the first electrode 2. The current is removed and the phase change material 8 cools rapidly to a temperature below the crystallization temperature, which results in the portion 9 of the phase change material 8 covering the first electrode 2 having the amorphous state. To obtain a crystalline state (FIG. 1A), a lower current write pulse (a SET pulse) is applied to the phase change memory element 1 for a second period of time (typically longer in duration than the first period of time and the crystallization time of the amorphous phase change material) to heat the amorphous portion 9 to a temperature below its melting point, but above its crystallization temperature. This causes the amorphous portion 9 of the phase change material 8 to re-crystallize to the crystalline state that is maintained once the current is removed and the phase change memory element 1 is cooled. The phase change memory element 1 is read by applying a read voltage, which does not change the phase state of the phase change material 8.
Due to the low resistivity of crystalline phase change material, a large RESET current density may be required to provide sufficient power to melt the phase change material. A large current density may cause undesired electro-migration in conductive material and may cause phase separation in the phase change material. Additionally, weak adhesion between the phase change material and other layers may introduce long term reliability issues in the phase change memory element.